thus established, the total quantity of solid matter carried off in solution in a given time being much the same in one river as in another. Roughly speaking, it may be said that where the rainfall is greatest the solubility is least; where the rainfall is least the solubility is greatest.

It is needless to follow the details of the calculation by which the author is finally led to the conclusion that about 8,370,630 tons of solids are annually removed in solution by the rivers of England and Wales. Distributing the denudation equally over the country, the total area being 58,300 square miles, we obtain a general lowering of the surface to the extent of 000077 of a foot in a single year; in other words, it would require 12,978 years to reduce the surface of England and Wales by one foot through the solvent action of rain alone.

Fewer data exist for extending this interesting inquiry to the continent of Europe, and fewer still when we pass to other parts of the world. But, making the best of available data, and proceeding on the principle that 'Nature, on the whole, averages the results,' Mr. Reade feels justified in assuming provisionally that about 100 tons of rocky matter will be dissolved by rain from every English square mile of the solid surface of the earth in the course of a year.

All this dissolved matter, however far it may be transported by rivers, ultimately runs down into the sea. If then, as commonly supposed, the sea contains only what has been washed out of the land, the results previously attained may help us to form some crude idea of the length of time which has been needed to give the ocean its present composition. Not to be irksome, we may pass over an array of figures and a number of provisional assumptions, in order to reach conclusions of general interest. These conclusions are, that it would take, in round numbers, 20,000,000 years to accumulate the quantity of sulphates of lime and magnesia contained in the vast bulk of the ocean, but only 480,000 years to renew the carbonates of lime and magnesia; with reference, however, to the latter constituents, it must be borne in mind that a vast quantity of carbonate of lime is constantly being removed from sea water for the supply of the hard parts of shell-fish, crustaceans, corals, and other marine animals, and consequently the amount calculated as present in the ocean is far from indicating the total quantity which is poured into it. But what are we to say of the chlorides, especially the chloride of sodium which is the prime constituent of sea water? The ocean contains so much of this salt, and the rivers usually so little, that we are driven to conclude from the author's calculations that it would take 200,000,000 years to renew the chlorides in the ocean!

During the voyage of the Challenger' the specific gravity of the sea water was determined daily by Mr. J. Y. Buchanan. Over

1,800 samples were thus examined, representing a wide range of localities and very various depths. It is obvious that these determinations are of great interest, since the density of the water may be taken approximately as an index of its saltness. If, therefore, we lay down upon a chart the results of the investigation, some idea may be formed of the distribution of salt in the ocean. This has been done by Mr. Buchanan, who submitted his results to the Royal Geographical Society at their meeting on the 12th of March.

Great care was taken to secure accuracy in the determinations, and it is believed that the results obtained may be relied upon to the fifth place of decimals. As temperature has a great effect upon the density of a liquid, due care was taken to eliminate errors arising from this source. The samples taken from great depths were stored in the laboratory for four-and-twenty hours, in order to attain to the temperature of the atmosphere before examination.

The highest specific gravity was found in the Atlantic, where the water in certain areas was so concentrated as to have a density varying from 1.0275 to 1.0280. On both sides of the area of heavy salt water, the density fines off, and becomes lowest in the equatorial region, where it is reduced to about 1.0260 to 1.0265. The areas of maximum density coincide with regions of dry winds; it is clear that if the wind blow from a cold to a hot zone, it becomes comparatively drier as its temperature rises, and can consequently take up more moisture; hence such a wind sweeping across the ocean tends to concentrate the water beneath, and the greatest density was therefore found where constant dry winds prevailed. In this way the trade winds produce two regions of concentrated water; and as the trades are more developed in the Atlantic than in the Pacific, we find the areas of greatest density in the Atlantic. On the other hand, winds which blow from hot to cold climates soon get saturated, and, evaporation being then diminished, the water below remains comparatively dilute. A heavy rainfall also produces dilution of the water. Nor is the effect of ice to be ignored in this inquiry. During the formation of ice the water which separates in a solid form contains but little salt, and therefore the water left behind in a liquid state becomes comparatively concentrated.

Looking at the general results of Mr. Buchanan's inquiry, we observe two great zones in which water is concentrated by cold, one in the Arctic and the other in the Antarctic regions; then there are two areas in which concentration is effected by the trade winds, and here again one is situated in the northern and one in the southern hemisphere; between the cold areas and the regions of the trades are two intermediate zones with fresher water; and finally between the two belts of the trade winds there is a zone of dilute water corresponding with the region of calms, the equatorial water being in fact the freshest in any part of the ocean.

Every antiquary is familiar with the peculiar change which glass suffers when long exposed to atmospheric influences or buried in moist ground. The surface frequently becomes iridescent, and exhibits a marked tendency to exfoliate or peel off in delicate scales. Even those who are not antiquaries must have been attracted by the beauty of this iridescence, commonly exhibited on the surface of the socalled lachrymatories and other ancient vessels frequently found in Roman sepulchres. Glass exposed to ammoniacal exhalations will likewise become iridescent; and brilliant examples may not urcommonly be seen on panes of glass in the windows of stables. The chemical nature of this alteration is, however, by no means well understood. It may therefore be worth while to call attention to some communications on the subject recently presented to the French Academy of Sciences.8

A curious incipient change exhibited by glass, while retaining its transparency, has been detected and investigated by M. de Luynes. The surface of the glass in question appears on casual examination to be unaltered; but viewed under proper incidence of light it exhibits striations, and when slowly heated the exterior exfoliates. If placed in hot water, the liquid penetrates the fissures, finding its way from the circumference towards the centre of each scale, the edges of which thus become raised while the centre may remain fixed. This experiment indicates the way in which the surface of glass may naturally peel off. Had the glass under examination been exposed to atmospheric influences so that its disintegration could have proceeded naturally, it is obvious that moisture penetrating the fissures would have thrown off thin flakes, such as we see in ancient specimens. The scales loosened from the glass by artificial means were analysed, and their chemical composition compared with that of the unaltered portion; in one case the scales contained 77.8 per cent. of silica, whilst the glass from which they were taken yielded only

65 per cent. ; in another case the scales gave 78.4 per cent. of

silica against 68 per cent. in the unaltered glass. This comparative richness in silica appears to be due. to removal of aikalis from the original glass during its decomposition. Such an explanation is quite in accord with the results obtained by MM. Frémy and Clémandot in the paper about to be noticed.

For many years past these investigators have studied the properties of glass-one of them in the chemical laboratory, the other in the manufactory-and have already published interesting results in connection with the famous Venetian product known as aventurineglass. Their present paper deals chiefly with the artificial production

Recherches sur l'irisation du Verre.' Par MM. Frémy et Clémandot. Comptes Rendus, No. 5, 1877, p. 209.

'Note sur certaines altérations du Verre.' Par M. V. de Luynes. Ibid. No. 7, p. 303.

of iridescence on the surface of glass. They find that by exposing certain varieties to the action of water containing 15 per cent. of hydrochloric acid, under the combined influence of heat and pressure, the surface may be caused to acquire a beautiful iridescence which, unlike that on ancient glass, does not scale off, but remains adherent, thus permanently giving the glass much the appearance of mother-ofpearl. Many varieties of glass lend themselves with readiness to this treatment, whilst others remain unaffected. Here then is a test which may possibly admit of practical application in selecting glass for certain uses in the arts. However beautiful the iridescence may be, it is clearly undesirable that glass used for domestic purposes should be thus decomposed. For although the alteration to which we have referred has been brought about under exceptional conditions of temperature and pressure, there is no doubt that it would proceed to a limited extent even under normal conditions. Hence glass which is found to be easily acted upon should not be employed for bottles intended to hold acid liquids, like wine.

Although the electric conductivity, or power which different substances possess of transmitting electricity, has been determined with considerable accuracy in the case of metals and some other solids, it has been found much more difficult to extend the investigation to liquid conductors. One important source of inaccuracy is introduced by the phenomenon termed polarisation; that is to say, when a current of electricity is sent through a liquid, the metal plates between which the current passes become coated with the products of decomposition of the liquid, and this so-called 'polarisation of the electrodes' produces a diminution of current. Mercury being a metal is an excellent conductor, but other liquids offer vastly greater resistance than that of the metals. Water, for instance, is known to possess very low electric conductivity, or, what comes to the same thing, a very high specific resistance. It is curious, however, to note the enormous difference in the results obtained by different experimenters on this subject. To take extreme cases, the electric conductivity of water, as determined by Pouillet, is about sixty times as great as that determined by Magnus; whilst other results lying between these extremes, but differing one from another, have been deduced by Becquerel, Oberbeck, Rossetti, and Quincke. With such discordant results on record, it is clear that Professor Kohlrausch has done good service by investigating the subject afresh.9

As it is known that the presence of even a minute proportion of foreign matter greatly affects the conductivity of water, every

'Ueber das elektrische Leitungsvermögen des Wassers und einiger anderer schlechter Leiter.' Von F. Kohlrausch. Poggendorff's Annalen, Ergänzungsband viii. p. 1.

precaution was taken in these experiments to obtain the liquid in as pure a state as possible. The water was twice distilled with the utmost care, and allowed to come in contact with nothing but air and platinum. The apparatus in which the resistance was determined consisted of a hemispherical vessel of platinum, which served as one of the electrodes, while the other was a similar though smaller vessel placed within the first, but of course without touching it, the space between the two vessels being occupied by the liquid under examination. Precautions were also taken to avoid polarisation, by which the resistance might appear to be affected. The conductivity of water, purified and tested in this way, was found to be about half as great as that determined by Magnus, and only one hundred and twentieth of that obtained by Pouillet. To show the great resistance of such water, we may remark that silver conducts electricity almost a billion times better. If the water be allowed to remain for some hours in the platinum vessel, the conductivity of the liquid is considerably increased. When the water was condensed in a worm of silver instead of platinum, the conducting power was raised; and when glass was employed, it rose to five times that of the liquid condensed in platinum-a result attributed to the action of the water upon the glass and consequent contamination of the liquid. Rainwater collected in Darmstadt possessed a conductivity about twentyfive times as great as that obtained with the purest water. Snowwater appears to be purer than rain-water, for its conductivity was found to be much less.

Whilst water is frequently classed among conductors of electricity, alcohol and ether have been regarded as non-conductors or as semiconductors. It has been said, indeed, that water conducts 204 times better than alcohol. Professor Kohlrausch, however, has found that in several cases commercial absolute alcohol conducted better than pure water; the conductivity of the spirit being, in fact, two and a half times that of the purest water.

In the course of last year no fewer than twelve minor planets were discovered, the last having been No. 169, named Zelia, which was detected on the 28th of September. With the beginning of the year fresh discoveries were made, and three new planets have already been announced from French observatories.10 On the 10th of January M. Perrotin, of Toulouse, who detected Erigone a year ago, discovered the new planetoid No. 170; and the same body was found about ten days afterwards by Professor Peters, of Clinton, U.S. A planet, believed to be distinct from this, and therefore to be distinguished as No. 171, was discovered by M. Borrelly, of Marseilles, on the 13th of January; and the same observer detected another (No.

10 Découvertes des trois petites Planètes et d'une Comète, faites à Toulouse et à Marseille.' Comptes Rendus, No. 7, 1877.